Low voltage signal stripping circuit for an RFID reader

ABSTRACT

A reader of an RFID system has excitation circuitry for generating a higher voltage excitation signal, receiving circuitry for reading a lower voltage transponder signal and an antenna coupled with the excitation circuitry for transmitting the excitation signal and coupled with the receiving circuitry for receiving the transponder signal. The receiving circuitry includes a low voltage signal stripping circuit coupled with the antenna for isolating the transponder signal from the excitation signal preliminary to the receiving circuitry reading the transponder signal. The bulk of the components of the low voltage signal stripping circuit are low voltage components, which can be included in an application specific integrated circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of application Ser. No. 10/728,735,now U.S Pat No. 7,124,942, filed Dec. 5, 2003.

TECHNICAL FIELD

The present invention relates generally to radio frequencyidentification systems, and more particularly, to a low voltage signalstripping circuit for a reader of a radio frequency identificationsystem.

BACKGROUND OF THE INVENTION

Radio frequency identification (RFID) systems generally consist of atleast one host reader and a plurality of passive transponders, which arecommonly termed credentials, cards, tags, or the like. An essentialfunction of the host reader is to “excite” or power up the transponder.The reader transmits high voltage excitation signals into thesurrounding space, which are received by a transponder proximal to thereader and which provide operational electrical power for the internalelectronics of the recipient transponder. Once the transponder ispowered up, it communicates information to the reader in a contactlessmanner. In particular, the powered up transponder transmitscommunication signals in the form of electromagnetic waves into thesurrounding space which are received by the reader. Accordingly, all ofthe transponders and readers each have a transmitting function and areceiving function.

The transmitting and receiving functions may be performed by separatelydedicated discrete electronic components, but more commonly at leastsome of the electronic components are combined into integrated circuitsor shared between different functions within the transponder or readerto reduce manufacturing costs. For example, the internal electronics ofthe transponder may be limited to a single antenna for transmittingtransponder signals and receiving excitation signals and to anintegrated circuit for performing the remaining necessary operationalfunctions of the transponder.

A low-cost reader may desirably include an application specificintegrated circuit (ASIC), which is an integration of low voltage-ratedcomponents exclusively, since low voltage-rated components are generallysignificantly less expensive than high voltage-rated components.Nevertheless, a reader incorporating an ASIC is only cost-effective ifthe number of additional electronic components needed to complete theinternal electronics of the reader are minimized. A low-cost reader mayalso desirably employ a single antenna to perform both the excitationsignal transmitting function and the transponder signal receivingfunction. However, this results in the superposition of the smallervoltage transponder signal on the higher voltage excitation signal whichrequires standoff of the high voltage excitation signal from the lowvoltage-rated components included within the internal electronics of thereader.

Standoff of the high voltage excitation signal is generally the singlemost difficult function to incorporate into an ASIC of the readerbecause multiple high voltage-rated components, which are not permittedin an ASIC, are usually required for the standoff function. Accordinglystandoff is typically performed by special circuitry upstream of theASIC, which strips the transponder signal off the excitation signalbefore the transponder signal is amplified and detected by the reader.Many prior art techniques for stripping the transponder signal utilizespecial diode detection circuitry, which usually requires several highvoltage-rated components (e.g., diodes, resistors and capacitors) totolerate the relatively high voltage across the reader antenna. Suchhigh voltage-rated components are relatively expensive and cannot beincorporated into an ASIC as noted above.

The present invention recognizes a need for alternate low-cost receivingcircuitry in a reader which effectively strips a low voltage transpondersignal off of a high voltage excitation signal. Accordingly, it is anobject of the present invention to provide a low-cost, effective lowvoltage signal stripping circuit. More particularly, it is an object ofthe present invention to provide a low voltage signal stripping circuitwhich requires a minimal number of high voltage components. It isanother object of the present invention to provide a low voltage signalstripping circuit having a plurality of low voltage components which areincorporated into an ASIC.

These objects and others are accomplished in accordance with theinvention described hereafter.

SUMMARY OF THE INVENTION

The present invention is a low voltage signal stripping circuit for areader of an RFID system. The low voltage signal stripping circuitcomprises an inlet resistor, an amplifier, a feedback circuit and a DCshift voltage or a DC shift current source. The inlet resistor ispreferably a high voltage component and the amplifier and the feedbackcircuit are preferably low voltage components. The amplifier and thefeedback circuit are preferably included in an application specificintegrated circuit. The feedback circuit preferably has a pathwayincluding a pair of clamping diodes aligned in series and a pathwayincluding a feedback resistor having a resistance. More particularly,the feedback circuit preferably has a first pathway including a firstpair of clamping diodes aligned in series in a first direction, a secondpathway including a second pair of clamping diodes aligned in series ina second direction opposite the first direction, and a third pathwayincluding a feedback resistor. The inlet resistor preferably has aresistance about equal to the resistance of the feedback resistor.

The low voltage signal stripping circuit preferably further comprises asumming node positioned upstream of the amplifier and downstream of theinlet resistor, feedback circuit and DC shift voltage or DC shiftcurrent source to sum outputs from the inlet resistor, the feedbackcircuit and the DC shift voltage or DC shift current source. Theamplifier has a first input coupled with the inlet resistor and a secondinput tied to a reference voltage. The first input of the amplifier ispreferably an inverting negative input, while the second input of theamplifier is preferably a non-inverting positive input.

In accordance with another embodiment, the present invention is a readerof an RFID system comprising excitation circuitry for generating ahigher voltage excitation signal, receiving circuitry for reading alower voltage transponder signal, and an antenna which is coupled withthe excitation circuitry for transmitting the excitation signal andwhich is coupled with the receiving circuitry for receiving thetransponder signal. The receiving circuitry includes the above-recitedlow voltage signal stripping circuit. The low voltage signal strippingcircuit is coupled with the antenna for isolating the transponder signalfrom the excitation signal preliminary to reading the transponder signalwith the receiving circuitry. The inlet resistor of the low voltagesignal stripping circuit is preferably positioned in series between theantenna of the reader and the amplifier of the low voltage signalstripping circuit.

In accordance with another embodiment, the present invention is a lowvoltage signal stripping circuit for a reader of an RFID system which isalternately characterized from the above-recited low voltage signalstripping circuit. The present low voltage signal stripping circuit ischaracterized as comprising means for creating a low voltage outputsignal from a high voltage antenna signal which is input to the lowvoltage signal stripping circuit. The low voltage stripping circuitfurther comprises means for creating a DC shift voltage or a DC shiftcurrent, means for selectively distributing a feedback signal from anamplifier having an output operating range, means for creating a summedsignal by summing the low voltage output signal, the selectivelydistributed feedback signal, and the DC shift voltage or DC shiftcurrent, and means for inputting the summed signal to an amplifier inputof the amplifier.

The output operating range of the amplifier has an upper voltage limit.The low voltage output signal preferably has a low voltage value whichis below the upper voltage limit of the output operating range. The highvoltage antenna signal preferably has a high voltage value which isabove the upper voltage limit of the output operating range of theamplifier.

The amplifier preferably has a second amplifier input in addition to theabove-recited amplifier input. The low voltage signal stripping circuitpreferably further comprise means for creating a reference voltage andmeans for inputting the reference voltage to the second amplifier input.

In accordance with another embodiment, the present invention is a methodfor processing a high voltage antenna signal waveform including a lowvoltage transponder signal containing readable information superposed ona high voltage excitation signal. The method comprises the steps ofproviding the high voltage antenna signal waveform, specifying alocation on the waveform where a waveform portion containing thereadable information is to be isolated, specifying a size of thewaveform portion to be isolated at the specified location on thewaveform, isolating the waveform portion, and reading the readableinformation on the waveform portion. The location on the waveform ispreferably specified by specifying a relative voltage value on thewaveform. The size of the waveform portion is preferably specified byspecifying an absolute voltage range applied to the waveform at thespecified location.

In accordance with another embodiment, the present invention is a methodfor processing a high voltage antenna signal waveform comprising thesteps of receiving a high voltage antenna signal containing readableinformation from an antenna at an input of a low voltage strippingcircuit. A voltage of the high voltage antenna signal is limited tocreate a low voltage output signal containing the readable information.The voltage of the high voltage antenna signal preferably exceeds anupper voltage tolerance of the amplifier, while the low voltage outputsignal preferably has a voltage below an upper voltage tolerance of theamplifier. The voltage of the high voltage antenna signal is preferablylimited by passing the high voltage antenna signal through an inletresistor. The method further comprises creating a DC shift voltage or aDC shift current and selectively distributing a feedback signal from anamplifier having an output operating range. The low voltage outputsignal, selectively distributed feedback signal, and DC shift voltage orDC shift current are summed to create a summed signal containing thereadable information. The summed signal is passed through the amplifierto create an amplifier output signal containing the readableinformation. The readable information on the amplifier output signal isthen read.

The amplifier preferably has a first input, which is more preferably aninverting input, and a second input, which is more preferably anon-inverting input. The first and second inputs are preferably held toessentially a same voltage value. The summed signal is input to thefirst input and a voltage reference is input to the second input. Theamplifier preferably provides a gain to the summed signal in response tothe selectively distributed feedback signal to create the amplifieroutput signal. More particularly, the gain preferably varies as afunction of a voltage of the feedback signal distributed to the summedsignal. The gain is reduced when the voltage of the feedback signaldistributed to the summed signal is outside a predetermined voltagerange.

The feedback signal is preferably selectively distributed by a feedbackcircuit. The feedback circuit has a first pathway with a low impedanceto the feedback signal when a voltage of the feedback signal is above anupper limit of a predetermined voltage range, a second pathway with alow impedance to the feedback signal when the voltage of the feedbacksignal is below a lower limit of the predetermined voltage range, and athird pathway with a low impedance to the feedback signal when a voltageof the feedback signal is within the predetermined voltage range.

The present invention will be further understood from the drawings andthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a radio frequency identificationsystem employing a low voltage signal stripping circuit of the presentinvention.

FIG. 2 is a conceptualized circuit diagram of the low voltage signalstripping circuit of FIG. 1.

FIG. 3 is a conceptualized circuit diagram of an alternate embodiment ofthe low voltage signal stripping circuit of FIG. 1.

FIG. 4 is a schematic view of a waveform of a high voltage antennasignal input to the low voltage signal stripping circuit.

FIG. 5 is a schematic view of a waveform of a low voltage amplifieroutput signal output from the low voltage signal stripping circuit.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a radio frequency identification (RFID) systememploying an embodiment of the low voltage signal stripping circuit ofthe present invention is shown and generally designated 10. The RFIDsystem 10 comprises a reader 12 and a transponder 14. The reader 12 ispreferably an active device and the transponder 14 is preferably apassive device.

An “active device” is defined herein as an electrically powered devicewhich has an internal electrical power supply, such as a rechargeable ordisposable battery, or is hard wired to an external electrical powersupply. The electrical power supply is physically coupled with theactive device to directly supply essentially all of the electrical powerrequired to operate the active device. Thus, the active device iscontinuously operational for its intended purpose upon being physicallycoupled with the electrical power supply.

A “passive device” is also an electrically powered device, but is notphysically coupled with an electrical power supply. The electrical powerrequired to operate the passive device is indirectly supplied to thepassive device by electromagnetic waves, which are propagated throughopen space to the passive device from a remote source. Thus, the passivedevice is only intermittently operational for its intended purpose,wherein the operational state of the passive device is a function ofwhether the passive device is receiving electromagnetic waves ofsufficient strength.

In the present RFID system 10, the remote source of electromagneticwaves is the reader 12. The electromagnetic waves transmitted from thereader 12 typically have a limited range due to size and powerconstraints of the reader 12. Thus, the reader 12 and transponder 14 ofthe RFID system 10 are simultaneously operational only when thetransponder 14 is within the range of the reader 12 and, moreparticularly, when the reader 12 and transponder 14 are positioned inrelative proximity to one another such that the transponder 14 receiveselectromagnetic waves of sufficient strength from the reader 12 to powerup the transponder 14.

In most conventional RFID systems, the position of the reader isstationary (i.e., constant) relative to the surrounding environment,while the position of the transponder is portable (i.e., variable)within the surrounding environment. In such cases, the user of the RFIDsystem moves the portable transponder into relative proximity with thestationary reader to enable simultaneous operation of the both thetransponder and reader. In some conventional RFID systems, however, theposition of the reader may be portable relative to the surroundingenvironment, while the position of the transponder is either portable orstationary. In the case of a portable reader and a stationarytransponder, the user moves the portable reader into relative proximitywith the stationary transponder to enable simultaneous operation of theboth the transponder and reader. In the case of a portable reader and aportable transponder, the user may move both the portable reader and theportable transponder into relative proximity with one another to enablesimultaneous operation of the both the transponder and reader. Thepresent invention is not limited to any one of the above-recited RFIDsystem configurations.

The electromagnetic waves, which are transmitted from the reader 12through open space to the transponder 14 for electrically powering upthe transponder 14, are termed “excitation signals.” The reader 12comprises a plurality of functional elements including excitationcircuitry 16 for generating an excitation signal 18 and a reader antenna20 for transmitting the excitation signal 18 into the open space of theexternal environment 22 surrounding the reader 12. The excitation signal18 is characterized as a high voltage signal preferably having arelatively high voltage within a range of about 75 to 125 volts zero topeak. The reader 12 preferably further comprises a tuning capacitor 23for the reader antenna 20.

The transponder 14 likewise comprises a plurality of functional elementsincluding a transponder antenna 24 for receiving the excitation signal18 and transponder circuitry 26 for generating a communication signaltermed a transponder signal 28. The transponder circuitry 26 is housedwithin the transponder 14 and is coupled with the transponder antenna24, which communicates electrical power resulting from reception of theexcitation signal 18 to the transponder circuitry 26. The transponderantenna 24 has an excitation signal reception range which is generallyabout 4 to 5 inches when the reader antenna 20 and transponder antenna24 are coaxially aligned. When the transponder 14 and/or reader 12 ismoved to a proximal position such that the distance between reader 12and transponder 14 is within the excitation signal reception range ofthe transponder antenna 24, the transponder antenna 24 receives theexcitation signal 18 at a sufficient strength to power up thetransponder circuitry 26, thereby activating the transponder 14.

Upon activation, the transponder circuitry 26 generates the transpondersignal 28, which contains readable information copied or otherwisederived from the memory of the transponder circuitry 26. The transpondersignal 28 is a communication signal in the form of an electromagneticwave like the excitation signal 18. It is noted that communicationsignals of RFID systems (i.e., excitation and transponder signals) aretypically termed radio frequency signals. However, the excitation andtransponder signals 18, 28 of the present invention are not limitedexclusively to signals having specific frequencies within the narrow“radio frequency” range, as “radio frequency” is commonly defined forthe radio communication industry.

The transponder 14 transmits the transponder signal 28 into the openspace of the external environment 22 via the transponder antenna 24. Thetransponder signal 28 is characterized as a low voltage signalpreferably having a relatively low voltage within a range of about 1 to100 millivolts at the reader antenna 20. The transponder antenna 24, asdescribed and shown, is a single antenna which performs both thereceiving and transmitting functions of the transponder 14. Inparticular, the transponder antenna 24 receives the excitation signal 18from the external environment 22 and transmits the transponder signal 28into the external environment 22. Thus, the transponder antenna 24 istermed a “dual-function antenna.” However, the present invention is notlimited to an RFID system having a transponder with a singledual-function transponder antenna. The present invention alternatelyencompasses an RFID system having a transponder with two separatetransponder antennas, which separately perform the receiving andtransmitting functions of the transponder.

The reader antenna 20 is likewise shown as a single dual-functionantenna which performs both the receiving and transmitting functions ofthe reader 12. In particular, the reader antenna 20 receives thetransponder signal 28 from the external environment 22 and transmits theexcitation signal 18 into the external environment 22. Accordingly, theexcitation signal 18 generated by the excitation circuitry 16 and thetransponder signal 28 generated by the transponder circuitry 26 may bothbe on the reader antenna 20 at the same time with the low voltagetransponder signal 28 superposed on the high voltage excitation signal18.

An exemplary transponder having utility in the RFID system 10 of thepresent invention is disclosed in U.S. Pat. Nos. 4,730,188 and5,541,574, incorporated herein by reference. The disclosed exemplarytransponder consists of a single dual-function transponder antenna forreceiving an excitation signal and transponder circuitry for generatinga transponder signal. The entire transponder circuitry is disclosed asbeing wholly integrated within a single integrated circuit chip. Anexemplary dual-function reader antenna and exemplary excitationcircuitry having utility in the reader of the RFID system 10 arelikewise disclosed in U.S. Pat. Nos. 4,730,188 and 5,541,574.

In addition to the excitation circuitry 16 and reader antenna 20, thereader 12 further comprises receiving circuitry 30 for reading thereadable information contained in the transponder signal 28 received bythe reader antenna 20 in accordance with any number of techniques wellknown to the skilled artisan such as, for example, disclosed in U.S.Pat. No. 4,730,188. The receiving circuitry 30 includes a low voltagesignal stripping circuit 34. Both the low voltage signal strippingcircuit 34 and excitation circuitry 16 are coupled in parallel with thereader antenna 20. Accordingly, the function of the low voltage signalstripping circuit 34 is to isolate the transponder signal 28 from theexcitation signal 18 preliminary to reading the transponder signal 28.Most of the receiving circuitry 30 and excitation circuitry 16 isincluded within an application specific integrated circuit (ASIC) 32,which is an active circuit powered by the external or internal powersupply (not shown) of the reader 12.

Referring additionally to FIG. 2, the low voltage signal strippingcircuit 34 comprises a high voltage inlet resistor 36, an operationalamplifier 38, a first pair of clamping diodes 40, 42, a second pair ofclamping diodes 44, 46, a feedback resistor 48, a DC shift voltagesource 50, a DC shift voltage resistor 52, and a stripping circuit input54. The stripping circuit input 54 is coupled with the reader antenna 20and the inlet resistor 36 is positioned in series downstream of thestripping circuit input 54. The inlet resistor 36 is selected to isolatethe internal circuitry of the ASIC 32 from high voltage antenna signalsgenerated by the reader antenna 20 and transmitted to the strippingcircuit input 54. As such, the inlet resistor 36 preferably has aresistance about equal to the resistance of the feedback resistor 48.

The operational amplifier 38 is a differential amplifier having anegative input 56 and a positive input 58. The negative input 56 isinverting (i.e., a negative input signal creates a positive outputsignal), while the positive input 58 is non-inverting (i.e., a positiveinput signal creates a positive output signal). The positive input 58 ofthe operational amplifier 38 is tied to V_(ref), while the negativeinput 56 is coupled with a summing node 62, which sums a DC shiftvoltage from the DC shift voltage source 50, the inlet resistor outputsignal from the inlet resistor 36, and a feedback signal from a feedbackcircuit 64 of the operational amplifier 38.

The feedback circuit 64 consists of the first pair of clamping diodes40, 42, the second pair of clamping diodes 44, 46, and the feedbackresistor 48, all positioned in parallel to one another. Both diodes ofthe first pair of clamping diodes 40, 42 are aligned in series in afirst direction, while both diodes of the second pair of clamping diodes44, 46 are aligned in series in a second direction opposite the firstdirection.

It is noted that the inlet resistor 36, which is external to the ASIC32, is the only component of the low voltage signal stripping circuit 34that must standoff the relatively high voltage antenna signals from thereader antenna 20. Standard resistors are generally capable of standingoff such high voltages. All remaining components of the low voltagesignal stripping circuit 34 are relatively inexpensive low voltagecomponents, which can be incorporated into the ASIC 32. Significanteconomies are realized in the production of the resulting reader 12 byincorporating the majority of the low voltage signal stripping circuit34 into the ASIC 32.

A method of operating the low voltage stripping circuit 34 is describedhereafter with continuing reference to FIGS. 1 and 2. Operation of thelow voltage signal stripping circuit 34 is initiated when the highvoltage antenna signal is received at the stripping circuit input 54from the reader antenna 20. The high voltage antenna signal is awaveform which has the characteristics of a sine wave and whichtypically has a voltage exceeding the voltage tolerances of the ASIC 32.The high voltage antenna signal comprises the low voltage transpondersignal 28 superposed on the high voltage excitation signal 18.

The high voltage antenna signal passes through the inlet resistor 36 andis output as an inlet resistor output signal at the summing node 62. Afunctional characteristic of the operational amplifier 38, whenoperating in cooperation with the feedback resistor 48, is that theinverting input 56 is held to the same voltage value as the voltagevalue V_(ref) of the non-inverting input 58. V_(ref) is generally aboutone half of the supply voltage of the operational amplifier.Accordingly, V_(ref) is preferably about 2.5 volts. As a result, all thehigh voltage of the antenna signal is dropped across the inlet resistor36 and the voltage range of the inlet resistor output signal does notexceed the output operating range of the operational amplifier 38, whichis typically between about 0 and +5 volts.

In general terms, the inlet resistor output signal is processed in thelow voltage signal stripping circuit 34 by a shifting technique and aclamping technique to specify and isolate a desired portion of the highvoltage antenna signal waveform to be read by the reader 12. Inparticular, the shifting technique specifies a desired location on thewaveform where the waveform portion is to be isolated and the clampingtechnique specifies the size of the waveform portion to be isolated atthe specified location on the waveform. More particularly, the shiftingtechnique specifies a desired relative voltage value on the waveform andthe clamping technique specifies a desired absolute voltage range whichis applied to the waveform at the specified voltage value.

The shifting technique is performed by creating a DC shift voltage usingthe DC shift voltage source 50, which may, for example, be theelectrical power supply of the reader 12 or a battery separate from theelectrical power supply of the reader 12. The DC shift voltage iscreated in accordance with any number of well known techniques withinthe purview of the skilled artisan. The DC shift voltage is preferablycreated in correspondence with a location (i.e., voltage value) on thewaveform where it is desired to apply the voltage range specified by theclamping technique in a manner described hereafter. As is apparent, thepractitioner can apply the specified voltage range to substantially anylocation on the waveform simply by varying the DC shift voltage.

The clamping technique is performed by the feedback circuit 64. Theclamping technique is initiated by conducting a feedback signal from afeedback node 66 at the output of the operational amplifier 38 to one ofa plurality of available conductive pathways through the feedbackcircuit 64. A first pathway 68 is through the first pair of clampingdiodes 40, 42, a second pathway 70 is through the second pair ofclamping diodes 44, 46, and a third pathway 72 is through the feedbackresistor 48. The ultimate pathway conducting the feedback signal throughthe feedback circuit 64 is a function of the voltage at the feedbacknode 66 and the properties of the components selected for the feedbackcircuit 64.

The first pair of clamping diodes 40, 42 is preferably selected so thatthe impedance of the first pathway 68 to the feedback signal is low whenthe voltage at the feedback node 66 is above an upper limit of apredetermined voltage range, thereby shorting out the feedback resistor48 of the third pathway 72. As a result, high voltage feedback signalsexceeding the upper limit of the predetermined voltage range are outputfrom the first pathway 68 of the feedback circuit 64 to the summing node62.

The second pair of clamping diodes 44, 46 is preferably selected so thatthe impedance of the second pathway 70 to the feedback signal is lowwhen the voltage at the feedback node 66 is below a lower limit of apredetermined voltage range, likewise shorting out the feedback resistor48 of the third pathway 72. As a result, low voltage feedback signalsbelow the lower limit of the predetermined voltage range are output fromthe second pathway 70 of the feedback circuit 64 to the summing node 62.

When the voltage at the feedback node 66 is at a desired voltage valuewithin the predetermined voltage range, the impedance of the thirdpathway 72 to the feedback signal is preferably such that feedbacksignals having the desired voltage value are output from the thirdpathway 72 of the feedback circuit 64 to the summing node 62.

The gain of the operational amplifier 38 is directly related to theratio of the feedback signal over the inlet resistor output signal. Whenthe feedback signal is outside the predetermined voltage range, theoperational amplifier 38 provides significantly diminished gain to thesignal inputted to the inverting input 56 of the operational amplifier38. This controls the voltage of the amplifier output signal withoutdriving the operational amplifier 38 into saturation, which is anundesirable non-linear condition. Thus, the feedback circuit 64 is ableto maintain the inverting input 56 at a low voltage while theoperational amplifier 38 is held in its linear operable range. As aresult, high voltage conditions are eliminated at the inverting input56, which would otherwise damage the operational amplifier 38.

By selecting the predetermined voltage range in correspondence with thepredicted voltage range of the transponder signal 28, the feedbackcircuit 64 only permits the operational amplifier 38 to experience arelatively narrow voltage range of the high voltage antenna signal,which preferably encompasses the transponder signal 28. Thus, thefeedback circuit 64 specifies the size of a waveform portion to be readby the reader 12. Consequently, the operational amplifier 38 enablessubsequent detection of variations in the transponder signal 28, whichare very small relative to the excitation signal and which are otherwisemasked by the voltage of the excitation signal. At the same time, thefeedback circuit 64 effectively prevents the operational amplifier 38from experiencing high voltages, thereby avoiding damage to theoperational amplifier 38, which is a low voltage component, caused byhigh voltages.

Performance of the shifting and clamping techniques provides the DCshift voltage and feedback signal at the summing node 62, which aresummed with the inlet resistor output signal. The resulting summedsignal is input to the operational amplifier 38 via the inverting input56, thereby producing an amplifier output signal at the amplifier output74. The amplifier output signal, which contains the readable informationof the transponder signal 28, is conveyed to the remaining downstreamreceiving circuitry 30 where the amplifier output signal undergoesdemodulation and detection in a conventional manner to read theinformation contained therein.

Referring to FIG. 3, an alternate embodiment of a low voltage signalstripping circuit is shown and generally designated 80. Elements of thelow voltage signal stripping circuit 80 which are identical to theelements of the low voltage signal stripping circuit 34 shown in FIG. 2are designated by the same reference characters. As such, the lowvoltage signal stripping circuit 80 comprises the high voltage inletresistor 36, the operational amplifier 38, the first pair of clampingdiodes 40, 42, the second pair of clamping diodes 44, 46, and thefeedback resistor 48. However, the low voltage signal stripping circuit80 substitutes a DC shift current source 82 for the DC shift voltagesource and resistor of the low voltage signal stripping circuit 34.

The DC shift current source 82 modifies performance of the shiftingtechnique, while achieving substantially the same result as DC shiftvoltage source and resistor described above. In particular, the DC shiftcurrent source 82, which may likewise be the electrical power supply ofthe reader 12 or a battery separate from the electrical power supply ofthe reader 12, creates a DC shift current in accordance with any numberof well known techniques within the purview of the skilled artisan. TheDC shift current is preferably created in correspondence with a levelrequired to cancel current flow into the inlet resistor 36 which resultsfrom the specified voltage range on the antenna signal waveform. As isapparent, the practitioner can apply the specified voltage range tosubstantially any location on the waveform simply by varying the DCshift current.

In accordance with an example of the present shifting and clampingtechniques, an antenna signal having a 75V peak, as shown in FIG. 4, isinput to the low voltage signal stripping circuit 34. The feedbackcircuit 64 specifies the size of the portion of the antenna signalwaveform to be read by the reader 12 as a portion of the waveform withina 4 Vf range. The DC shift voltage or DC shift current specifies thelocation on the waveform to which the 4 Vf range is applied as thepositive 75V peak of the waveform. This specified waveform portion isdesignated AB.

The voltage of the antenna signal is dropped across the inlet resistor36 of the low voltage signal stripping circuit 34. The resulting inletresistor output signal is summed with the DC shift voltage or DC shiftcurrent and the feedback signal and conducted to the inverting negativeinput 56 of the operational amplifier 38. The operational amplifier 38generates an amplifier output signal waveform at the amplifier output74, as shown in FIG. 5. The amplifier output signal does not exceed the0 to +5 volt output operating range of the operational amplifier 38 andis in a condition to be read by the reader 12.

While the forgoing preferred embodiments of the invention have beendescribed and shown, it is understood that alternatives andmodifications, such as those suggested and others, may be made theretoand fall within the scope of the invention.

1. A method for processing a high voltage antenna signal waveformcomprising the steps of: providing a high voltage antenna signalwaveform including a low voltage transponder signal containing readableinformation superposed on a high voltage excitation signal; specifying alocation on said waveform where a waveform portion is to be isolated;specifying a size of said waveform portion to be isolated at saidspecified location on said waveform; isolating said waveform portion,wherein said waveform portion contains said readable information; andreading said readable information on said waveform portion.
 2. Themethod of claim 1, wherein said location is specified by specifying arelative voltage value on said waveform.
 3. The method of claim 1,wherein said size of said waveform portion is specified by specifying anabsolute voltage range applied to said waveform at said specifiedlocation.
 4. A method for processing a high voltage antenna signalcomprising the steps of: receiving a high voltage antenna signalcontaining readable information from an antenna at an input of a lowvoltage stripping circuit; limiting a voltage of said high voltageantenna signal to create a low voltage output signal containing saidreadable information; creating a DC shift voltage or a DC shift current;selectively distributing a feedback signal from an amplifier having anoutput operating range; and creating a summed signal containing saidreadable information by summing said low voltage output signal, saidselectively distributed feedback signal, and said DC shift voltage orsaid DC shift current.
 5. The method of claim 4, wherein said highvoltage antenna signal has a voltage exceeding an upper voltagetolerance of said amplifier.
 6. The method of claim 4, wherein said lowvoltage output signal has a voltage below an upper voltage tolerance ofsaid amplifier.
 7. The method of claim 4, wherein said voltage of saidhigh voltage antenna signal is limited by passing said high voltageantenna signal through an inlet resistor.
 8. The method of claim 4,wherein said amplifier has a first input and a second input held to asubstantially matching voltage value, wherein said summed signal isinput to said first input and a voltage reference is input to saidsecond input.
 9. The method of claim 4, wherein said amplifier has aninverting input and a non-inverting input held to a substantiallymatching voltage value, wherein said summed signal is input to saidinverting input and a voltage reference is input to said non-invertinginput.
 10. The method of claim 4, wherein said amplifier provides a gainto said summed signal varying as a function of a voltage of saidfeedback signal distributed to said summed signal, said gain beingreduced when said voltage of said feedback signal distributed to saidsummed signal is outside said predetermined voltage range.
 11. Themethod of claim 4 further comprising passing said summed signal throughsaid amplifier to create an amplifier output signal containing saidreadable information.
 12. The method of claim 11, wherein said amplifierprovides a gain to said summed signal in response to said selectivelydistributed feedback signal to create said amplifier output signal. 13.The method of claim 11 further comprising reading said readableinformation on said amplifier output signal.
 14. A method for processinga high voltage antenna signal comprising the steps of: receiving a highvoltage antenna signal containing readable information from an antennaat an input of a low voltage stripping circuit; limiting a voltage ofsaid high voltage antenna signal to create a low voltage output signalcontaining said readable information; creating a DC shift voltage or aDC shift current; selectively distributing a feedback signal from anamplifier having an output operating range, wherein said feedback signalis selectively distributed by a feedback circuit having a first pathwaywith a low impedance to said feedback signal when a voltage of saidfeedback signal is above an upper limit of a predetermined voltagerange, a second pathway with a low impedance to said feedback signalwhen said voltage of said feedback signal is below a lower limit of saidpredetermined voltage range, and a third pathway with a low impedance tosaid feedback signal when a voltage of said feedback signal is withinsaid predetermined voltage range; and creating a summed signalcontaining said readable information by summing said low voltage outputsignal, said selectively distributed feedback signal, and said DC shiftvoltage or said DC shift current.
 15. The method of claim 14 furthercomprising passing said summed signal through said amplifier to createan amplifier output signal containing said readable information.
 16. Themethod of claim 15, wherein said amplifier provides a gain to saidsummed signal in response to said selectively distributed feedbacksignal to create said amplifier output signal.
 17. The method of claim15 further comprising reading said readable information on saidamplifier output signal.
 18. The method of claim 14, wherein saidamplifier has a first input and a second input held to a substantiallymatching voltage value, wherein said summed signal is input to saidfirst input and a voltage reference is input to said second input. 19.The method of claim 14, wherein said amplifier has an inverting inputand a non-inverting input held to a substantially matching voltagevalue, wherein said summed signal is input to said inverting input and avoltage reference is input to said non-inverting input.
 20. The methodof claim 14, wherein said amplifier provides a gain to said summedsignal varying as a function of a voltage of said feedback signaldistributed to said summed signal, said gain being reduced when saidvoltage of said feedback signal distributed to said summed signal isoutside said predetermined voltage range.